The human 5T4 antigen is expressed in numerous cancer types and is substantially absent from normal tissues. Recently, high affinity monoclonal antibodies have been developed that specifically bind to the 5T4 antigen and cytotoxic agents have been conjugated to the 5T4 antibodies to form antibody drug conjugates for use in the treatment of 5T4-positive cancer (U.S. Pat. Nos. 8,044,178 and 8,309,094). It follows then that assessment of 5T4 expression could be a useful approach for identifying patients with 5T4-positive cancer. One approach would be the detection of the 5T4 antigen on circulating tumor cells (CTCs) in cancer patients.
Circulating tumor cells have been observed in the peripheral blood of patients with epithelial-derived cancers at ultra low concentrations (Kraeft et al., Clin Cancer Res 10: 3020-3028, 2004). The number of these cells has been shown to correlate with outcome for cohorts of metastatic breast cancer patients with progressive disease at the time of sampling (Cristofanilli et al., N Engl J Med 351: 781-791, 2004). For this reason, their characterization is of considerable biomedical interest in order to understand how these cells can travel via the blood stream to anatomically distant sites and form metastatic disease. Consequently, identifying CTCs associated with 5T4-positive cancer could provide a valuable diagnostic tool for patient identification.
Currently, CTCs are detected and analyzed primarily through immunocytochemical markers such as EpCam and the use of nuclear staining with DAPI (4′,6-diamidino-2-phenylindole), a fluorescent stain that binds strongly to A-T rich regions in DNA. Although these approaches have been successful in enumerating and distinguishing CTCs, they differ from standard cytopathologic approaches as they omit the correlation with standard morphologic staining upon which diagnostic pathology is dependent. This creates difficulty in comparing CTCs to tumor cells from other sites obtained by routine diagnostic procedures. Although the ability to detect CTCs has the potential to aide in diagnostic and individualized treatment of cancer and efficacy of treatment, the understanding of the biology of CTCs could be improved by including standard cytopathologic methods. A need exists in the art to utilize detailed high resolution imaging of CTCs with conventional diagnostic pathology staining methods and bright-field microscopy to confer the potential of making a standard cytopathologic diagnosis of circulating 5T4-positive carcinoma cells and advancing the adoption of diagnosis using 5T4-positive CTCs in the clinic.